# Connection to OPC UA Server not possible error when activate ARSim monitoring

**URL:** <https://community.br-automation.com/t/connection-to-opc-ua-server-not-possible-error-when-activate-arsim-monitoring/10362>\
**Category:** B&R Software & Runtime\
**Tags:** opc-ua\
**Created:** [March 3, 2026, 10:35am UTC](https://community.br-automation.com/t/connection-to-opc-ua-server-not-possible-error-when-activate-arsim-monitoring/10362 "2026-03-03T10:35:35Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![alvin.lim](https://avatars.discourse-cdn.com/v4/letter/a/FF8800/32.png) [@alvin.lim](https://community.br-automation.com/u/alvin.lim)\
**Post date:** [March 3, 2026, 10:35am UTC](https://community.br-automation.com/t/connection-to-opc-ua-server-not-possible-error-when-activate-arsim-monitoring/10362/1 "2026-03-03T10:35:35Z")

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Hello Community,

One of our customer is facing issue when using monitor mode for Ladder Diagram.

I am able to use monitor mode with the same project without any errors. However, on his PC after he log in with the created user after activating monitor mode, he is getting this error message.

 ![image](https://europe1.discourse-cdn.com/brcommunity/original/2X/8/892f7d1d81e9342492b362240e8553b36dcbf05c.png)

Any idea what is causing this error and how to resolve it?

Thanks in advance.

Best regards,  
Alvin

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**Author:** ![tommi.piiparinen](https://dub1.discourse-cdn.com/brcommunity/user_avatar/community.br-automation.com/tommi.piiparinen/32/2183_2.png) [@tommi.piiparinen](https://community.br-automation.com/u/tommi.piiparinen)\
**Post date:** [March 3, 2026, 10:39am UTC](https://community.br-automation.com/t/connection-to-opc-ua-server-not-possible-error-when-activate-arsim-monitoring/10362/2 "2026-03-03T10:39:20Z")

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Most likely some other server running on the port 4840.

Some troubleshooting instructions from Automation Help. (In mapp View context but should not make a difference in this case).  
[Automation Help: No OPC UA connection to ARsim](https://help.br-automation.com/#/en/6/visualization%2Fmappview%2Fguides%2Ffaq%2Fruntime_system_faq%2Farsim_opcua.html)

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<div class="post-metadata">

**Author:** ![alvin.lim](https://avatars.discourse-cdn.com/v4/letter/a/FF8800/32.png) [@alvin.lim](https://community.br-automation.com/u/alvin.lim)\
**Post date:** [March 4, 2026, 1:54am UTC](https://community.br-automation.com/t/connection-to-opc-ua-server-not-possible-error-when-activate-arsim-monitoring/10362/3 "2026-03-04T01:54:28Z")

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Hello Tommi,

Thank you for your reply.

You were right, port 4840 was already being used by another service on his PC, which caused the conflict. After changing the OPC UA server port to 4841, the issue was resolved and the connection is now working properly.